1. Field of the Invention
The invention relates generally to devices used to separate construction and mining materials by size, and more specifically relates to screening plants, which use vibrating screens of varying meshes to separate matter poured onto the screens.
2. Description of the Related Art
Conventional screening plants have been in use for some time. Such machines are used to separate materials, such as road construction debris, gravel, soil, sand and recyclables. Examples of conventional screening plants are shown in many U.S. Patents, such as U.S. Pat. No. 5,106,490 to McDonald and U.S. Pat. No. 4,923,597 to Anderson et al.
Conventional screening plants include a wide upper screen which is angled relative to horizontal, onto which material is poured. The screen vibrates, causing pieces of matter that are larger than the apertures to slide down the angled screen onto a pile of larger pieces of matter that collect on one side of the machine. Matter that is smaller than the apertures in the upper screen drops through the apertures, typically onto a second angled screen with smaller apertures, to be separated further. Screening plants are often portable, permitting them to be transported to the location where excavation, mining or construction takes place.
The drive mechanism for most screening plants includes an internal combustion engine that powers a pump for pressurizing hydraulic fluid. An example of such a mechanism is disclosed in U.S. Pat. No. 4,237,000 to Read. The fluid is pumped to a hydraulic motor that rotates a driveshaft with an attached eccentric, vibrating the screen box. The screen box typically includes an attached stack of similarly angled, parallel screens with progressively smaller apertures on each lower screen. Therefore, only the finer particulate matter, such as sand, passes through the lowest screen layer. This finer particulate matter is often conveyed by an elevating conveyor from beneath the primary screen apparatus to a pile spaced from the machine.
Problems arise from the use of conventional screening plants. The materials are normally poured onto the upper screen layer by the bucket of an excavating loader. The buckets of excavating loaders used to pour the material into the screening plants have a minimum size. Therefore, the width of the screen box, which is the distance, W, in FIG. 1, should not be significantly less than the width of the smallest normal bucket, which is about five feet. However, the stresses induced in a single screen box that is greater than five feet wide by a full bucket of material is significant, often resulting in the frame members of the screen box bending or breaking. The length, L, of the screen box is shown in FIG. 2.
Improvements have been made to the conventional screen box to reduce damage by heavy materials. These improvements include central support members extending between the frame members of the upper screen and the frame members of a lower screen. Such a support member is shown in U.S. Pat. No. 4,256,572 to Read. The support member distributes, among the frames of lower screens, some of the stress applied to the upper screen's frame due to the weight of the material dropped thereon. These improvements have reduced the damage, but they have not eliminated it.
The conventional screens used on screening plants are also expensive to design and make. Such screens must have long, extremely strong screen box frame members. Additionally, each member must be continuous across the screen box, without seams which are subject to breakage under the stresses induced by the large loads. Furthermore, the drive system needed to reciprocate large screen boxes must be extremely robust and therefore expensive, including a single vibrating driveshaft extending the entire width of the screen.
When the width of a conventional screening plant is increased, there is a disproportionate decrease in available amplitude of oscillation (called "throw"), there is a disproportionate loss of the ability to screen large matter and the manufacturing cost increases disproportionately to the increase in width.
Therefore, the need exists for an improved screening plant including screens and screen frames that are less susceptible to damage and more cost efficient to build. Additionally, such a screening plant should be able to be made wider with only a proportional increase in cost, and no decrease in ability to screen large material.